On the basis of the existing 2G and 3G, the 3GPP launches an evolved packet system EPS. During the preliminary period of building an EPS network, if an operator already has a mature GERAN/UTRAN network, the operator may adopt an existing CS voice solution to provide voice services for the sake of CS (circuit switched) investment protection and LET/EPC deployment policy.
CSFB (CS Fallback) is one of the outcomes of the CS over PS task in 3GPP R8. The background of the task is that a radio module of an LTE and CS dual-mode terminal is a single radio mode, i.e., a dual-mode or multi-mode terminal having LTE and UTRAN/GERAN access capabilities cannot receive/transmit circuit domain service signals when using LTE access. A CSFB technology is evolved so as to enable the terminal to initiate CS services such as voice services under LTE access, to receive paging of CS services such as voice, and to correctly process ongoing PS services of the terminal in the LTE network.
During the preliminary period of building a TD-LTE network, if an operator already has a mature UTRAN/GERAN network, the operator may adopt an existing CS voice solution to provide voice service in combination with a TD-LTE network deploy policy so as to protect CS investments, while the TD-LTE network merely processes data services (including IMS data services). In this case, when adopting the CSFB technology, i.e., when an UE under the coverage of LTE is processing voice services, the terminal first falls back to the CS network to process voice services in the CS network, which thereby achieves the objective of reusing the existing CS device to provide traditional voice services for subscribers in the TD-LTE network.
EPS/LTE standard TS23.018 have specified a CS fallback procedure over SGs and a procedure for Mobile Terminating Roaming Forward call (MTRF). However, in the EPS/LTE network, the method for charging the CS fallback MTRF call (including IMS messages) is still a pending issue.